on the Martian surface today but only under certain conditions. No large standing bodies of liquid water exist because the atmospheric pressure at the Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life. Although many celestial bodies in the Solar System have a hydrosphere, Earth is the only celestial body known to have stable bo Extraterr… Water on Mars exists today almost exclusively as ice and underground, with a small amount present in the atmosphere as vapor. Some liquid water may occur transiently on the Martian surface today but only under certain conditions. No large standing bodies of liquid water exist because the atmospheric pressure at the surface averages just 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the global average temperature is far too low (210 K (−63 °C)), leading to either rapid evaporation or freezing. Features called recurring slope lineae are thought to be caused by flows of brine—hydrated salts. In July 2018, scientists from the Italian Space Agency reported the detection of a subglacial lake on Mars, 1.5 kilometres (0.93 mi) below the southern polar ice cap, and spanning 20 kilometres (12 mi) horizontally, the first evidence for a stable body of liquid water on the planet. Because the temperature at the base of the polar cap is estimated at 205 K (−68 °C; −91 °F), scientists assume that the water may remain liquid due to the antifreeze effect of magnesium and calcium perchlorates. The 1.5-kilometre (0.93 mi) ice layer covering the lake is composed of water ice with 10 to 20% admixed dust, and seasonally covered by a 1-metre (3 ft 3 in)-thick layer of CO2 ice. Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life. Although many celestial bodies in the Solar System have a hydrosphere, Earth is the only celestial body known to have stable bodies of liquid water on its surface, with oceanic water covering 71% of its surface, which is essential to life on Earth. The presence of liquid water is maintained by Earth's atmospheric pressure and stable orbit in the Sun's circumstellar habitable zone; however, the origin of Earth's water remains uncertain. The main methods currently used for confirmation are absorption spectroscopy and geochemistry. These techniques have proven effective for atmospheric water vapor and ice. However, using current methods of astronomical spectroscopy it is substantially more difficult to detect liquid water on terrestrial planets, especially in the case of subsurface water. Due to this, astronomers, astrobiologists and planetary scientists use habitable zone, gravitational and tidal theory, models of planetary differentiation and radiometry to determine the potential for liquid water. Water observed in volcanic activity can provide more compelling indirect evidence, as can fluvial features and the presence of antifreeze agents, such as salts or ammonia. Using such methods, many scientists infer that liquid water once covered large areas of Mars and Venus. Water is thought to exist as liquid beneath the surface of some planetary bodies, similar to groundwater on Earth. Water vapour is sometimes considered conclusive evidence for the presence of liquid water, although atmospheric water vapour may be found to exist in many places where liquid water does not. Similar indirect evidence, however, supports the existence of liquids below the surface of several moons and dwarf planets elsewhere in the Solar System. Some are speculated to be large extraterrestrial "oceans". Liquid water is thought to be common in other planetary systems, despite the lack of conclusive evidence, and there is a growing list of extrasolar candidates for liquid water. In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies. Water on Mars exists today almost exclusively as ice and underground, with a small amount present in the atmosphere as vapor. Some liquid water may occur transiently on the Martian surface today but only under certain conditions. No large standing bodies of liquid water exist because the atmospheric pressure at the surface averages just 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the global average temperature is far too low (210 K (−63 °C)), leading to either rapid evaporation or freezing. Features called recurring slope lineae are thought to be caused by flows of brine—hydrated salts. In July 2018, scientists from the Italian Space Agency reported the detection of a subglacial lake on Mars, 1.5 kilometres (0.93 mi) below the southern polar ice cap, and spanning 20 kilometres (12 mi) horizontally, the first evidence for a stable body of liquid water on the planet. Because the temperature at the base of the polar cap is estimated at 205 K (−68 °C; −91 °F), scientists assume that the water may remain liquid due to the antifreeze effect of magnesium and calcium perchlorates. The 1.5-kilometre (0.93 mi) ice layer covering the lake is composed of water ice with 10 to 20% admixed dust, and seasonally covered by a 1-metre (3 ft 3 in)-thick layer of CO2 ice. Scientists' consensus is that a layer of liquid water exists beneath Europa's surface, and that heat energy from tidal flexing allows the subsurface ocean to remain liquid. The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Scientists used